The present invention relates in general to catalyst reactors and in particular to a new and useful method and apparatus of automatically regenerating a catalyst.
In many chemical, petrochemical and synthetic fuel manufacturing processes a raw material feed stream is passed over a catalyst in a reactor to either produce a desired product or remove certain impurities. The state of the catalyst is of prime importance to these operations because economic operation of a reactor and consequently the plant, is very much affected by the catalyst state. Product conversion, yield and operating temperature are dependent upon the selectivity of the catalyst.
A catalyst's selectivity decays with time because of a number of factors such as presence of impurities and their concentration in the feed stream, types of byproducts from side reactions, duration of reactor operation, etc. For example in the selective hydrogenation of acetylene in an ethylene rich stream green oil, polymerization of green oil on the catalyst surface, etc. occurrs. These are deposited on the catalyst surface, thus reducing the surface area available for the reaction. Hence, acetylene hydrogenation to ethylene is reduced. The condition may also lead to a product which is off specification. Therefore, after operating a reactor with fresh or regenerated catalyst for some time, the catalyst is either regenerated by passing steam through the reactor or replaced by new catalyst whichever may be the case depending on a specific situation.
The state of the art for control of the selective hydrogenation of acetylene into ethylene in the ethylene rich stream is well documented in the following patents:
______________________________________ 1. U.S. Pat. No. 2,802,889 Frevel et al 2. U.S. Pat. No. 2,814,653 Hogan et al 3. U S. Pat. No. 3,113,980 Robinson et al 4. U.S. Pat. No. 3,153,679 Rottmayr 5. U.S. Pat. No. 3,471,582 Lupfer 6. U.S. Pat. No. 3,656,911 Hobbs 7. U.S. Pat. No. 3,972,804 McLaughlin et al 8. U.S. Pat. No. 4,236,219 Killebrew et al 9. U.S. Pat. No. 4,241,230 Drinkard 10. U.S Pat. No. 4,249,907 Callejas ______________________________________
These patents only teach the techniques for the temperature control of reactors. They do not provide for any indication of the current state of the catalyst. Consequently, any or all of the following situations may occur:
1. As catalyst selectivity decreases, less and less acetylene will be hydrogenated into ethylene. Thus, the probability for occurrence of product off-specification increases and ultimately may occur, thus resulting in loss of production.
2. The operator may increase operating temperature of the reactor to compensate for decline in catalyst selectivity on the basis of information from the manufacturer of the catalyst and acetylene concentration in the reactor exit stream. This presents a situation where an operator may be unable to discriminate among the causes for occurrences of more than normal acetylene concentration in the exit stream. This may be a result of several possible conditions, for example, sudden drop in feed temperature, sudden breakthrough of acetylene in cracking furnaces, malfunctioning of intermediate unit operations or their associated components, malfunctioning/failure of instruments, etc.
3. Increase in operating temperature of the reactor results in increasing energy consumption in the feed preheater, thus increasing coolant use in the inter catalyst bed cooler, and increases ethylene hydrogenation to ethane.
Moreover, in the current state of the art industrial practice, catalyst is regenerated either after occurrence of a severe product off specification or it has been scheduled by maintenance personnel on the reactor, on the basis of an elapsed period of operation or during plant shutdown. The same is applicable to catalyst change. Therefore, the reactor is normally operated with catalyst for a period longer than the recommended period between two successive regenerations and replacement by fresh catalyst. Therefore, one or more of the aforementioned three conditions may occur, or worse yet, the catalyst may be poisoned. The recommended period of operation between two successive catalyst regeneration is about 18 months. Continued operation without regeneration may poison the catalyst completly, thus reducing its life (normally about 5 years) and may require new catalyst.